Optical selectors apply a light to articles to be selected, such as falling particles, measuring light which is reflected and transmitted from the particles, and finally, if there is any abnormality in the surface color tone, detecting those articles or particles which have such a difference in light quality so that the abnormal particle or article is eliminated by means of a blast of air. In that case, there is a necessity of adequately dispersing the particles so as not to pile them up at the location at which the quality of light transmitted therethrough and reflected therefrom is measured. For that purpose, the articles to be selected are accelerated by means of a chute to prevent any build-up. The chute is installed at a vertical inclination, the articles to be measured being supplied at the upper part of the chute, the dead weight of the articles causing them to slide along the surface of the chute. However, if the bottom surface of the chute is simply planar, the articles to be selected may become concentrated only on one side of the chute during their decent.
In order to prevent such a tendency, it has been proposed to provide the chute with corrugations or grooves near the bottom. Even so, if a chute with simple corrugations is used, the downwardly flowing articles tend to become vertically piled up when the flow rate increases, and this presents problems in measurement of the light quantity. For example, if the articles to be selected become piled up, it becomes impossible to measure the quantity of reflected light from certain particles; similarly, if transmitted light is used, the detection of abnormal articles becomes difficult because of a decrease in the light quantity when the articles become piled up. Although such a problem rarely occurs if the articles to be selected have a shape which is approximately spherical, such a problem causes serious concerns particularly when the shape of the articles is flat or long and narrow.